The present invention relates to an audio amplifier circuit device which functions to automatically restrict other frequency signals than desired signals without manual selecting device and, more particularly, to an audio amplifier circuit device which amplifiers frequency signals corresponding to usual sounds without attenuation and, when noise components which may be high and/or low frequency exist, amplifies only desired components by automatically attenuating the high and/or low frequency components.
It should be understood that the term "audio amplifier circuit device" used herein means a device adapted to be connected between two acoustic transducers, e.g. a microphone and a loud speaker. An example of applications of this invention may be an acousticon.
In order to restrict undesired sound, it has been used to add a switch means to the amplifier circuit device, which, when it is used in an area where noises exist, is actuated to reduce the amplifier gain or to limit output power for the undesired sounds.
That is, the user of the sound collecting and reproducing device has to operate the selector switch means according to his own judgement of whether or not he is in a noisy environment and which frequency components are to be restricted. In a case where the sound collecting and reproducing device is the acousticon, the manual operation of the selector switch means becomes frequent and difficult because of the movement of the user himself which causes the noise to be not steady.